I ran into the same error after migrating Analysis Services from 2012 to 2014 and since this is the only post about it online, I figured I would share my experience. Adam was correct that simply scripting out the database, dropping it, and then recreating it would resolve the issue of backups throwing the error:

Backup and restore errors: Unexpected number of disk files associated with database object. Possible cause for that is missing or corrupt cryptokey.bin or detach.log file.